Process for conversion of natural gas to liquid hydrocarbons and a plant for carrying out the process

ABSTRACT

A process and plant for conversion of a feed hydrocarbon stream to liquid hydrocarbon products in a small scale GTL plant, comprising the use of a cryogenic air separation unit (ASU), optionally together with vacuum pressure swing adsorption (VPSA), an autothermal reformer (ATR) or catalytic partial oxidation (CPO), and pressure swing adsorption (PSA) unit to produce a synthesis gas for downstream Fischer-Tropsch (FT) synthesis for production of liquid hydrocarbons.

RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 14/885,550, filedOct. 16, 2015, the entire disclosure of which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a process for conversion of a feedhydrocarbon stream such as natural gas to liquid hydrocarbon products.More specifically, the invention relates to a process comprising the useof a cryogenic air separation unit (ASU), optionally together withvacuum pressure swing adsorption (VPSA), to provide oxygen with purityof at least 80% vol. The feed hydrocarbon stream is combined with steam,optionally with subsequent pre-reforming, and passed with the oxygenthrough an autothermal reformer (ATR) or catalytic partial oxidation(CPO) to produce a raw synthesis gas, of which a portion is passedthrough a pressure swing adsorption (PSA). The raw synthesis is used indownstream Fischer-Tropsch synthesis for production of liquidhydrocarbons, including production of a Fischer-Tropsch tail gas streamwhich is recycled to e.g. the feed hydrocarbon stream prior to or afteraddition of steam, or to the ATR or CPO. The invention further relatesto a plant for carrying out the process. The process and plant areparticularly suitable for small GTL plant(s) producing e.g. 500-5000 BPDof liquid hydrocarbons, in particular 1000-3000 BPD.

It is well-known to use autothermal reforming (ATR) for producingsynthesis gas from e.g. natural gas with downstream production of liquidhydrocarbons, since ATR technology is capable of producing synthesis gaswith the right H₂/CO molar ratio of about 2 required for downstreamFischer-Tropsch synthesis used for producing liquid hydrocarbons, inparticular diesel.

While the combination of ATR and Fischer-Tropsch is normally used inlarge plants, also called gas to liquid (GTL) plants where large naturalgas reserves exist, there is an increasing demand to produce liquidhydrocarbons from much smaller natural gas reserves, i.e. by providingsmall GTL plants. This development is at least partly driven by poorpipeline accessibility to the remote places where such smaller naturalgas reserves (fields) are located. Hence, it is desirable to be able toconvert natural gas to liquid hydrocarbons, as the latter is easier totransport. Apart from natural gas, associated gas, also calledassociated petroleum gas (APG), may also suitably be converted to liquidhydrocarbons, instead of being flared.

For small GTL plants producing about 1000 BPD (barrels per day) from anatural gas feed, it is known from U.S. Pat. No. 9,034,208 to combinethe use of vacuum pressure swing adsorption, autothermal reforming, andhydrogen removal by means of a hydrogen-membrane while using the tailgas from Fischer-Tropsch synthesis (Fischer-Tropsch tail gas) as fuel topreheat the feeds supplied to the autothermal reforming stage.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process and plantwhich is more energy efficient and more inexpensive than prior artprocesses, particularly for small GTL plants.

As used herein the term “small GTL plant(s)” means a plant capable ofproducing 500-5000 BPD of liquid hydrocarbons.

This and other objects are solved by the present invention.

We have found that by providing a cryogenic air separation unit incombination with autothermal reforming or catalytic partial oxidation,pressure swing adsorption of raw synthesis gas from the ATR as well asFischer-Tropsch tail gas recycle to the hydrocarbon feed or to the ATR,significant and unexpected advantages are obtained.

Accordingly, in one aspect of the invention there is provided a processfor the conversion of a feed hydrocarbon stream into liquid hydrocarbonscomprising:

(a) providing oxygen with a purity of at least 80% vol., preferably atleast 85% vol., more preferably at least 90% vol., most preferably atleast 95% vol., by using a cryogenic air separation unit (ASU);

(b) mixing steam to the feed hydrocarbon stream to form ahydrocarbon-steam stream;

(c1) adding the oxygen of step (a) and the hydrocarbon-steam stream ofstep (b) to an autothermal reformer (ATR), or catalytic partialoxidation (CPO) unit, or

(c2) combining the oxygen of step (a) with the resulting stream of step(b) and then adding to an autothermal reformer (ATR), or catalyticpartial oxidation (CPO) unit;

(d) withdrawing from the ATR or CPO a raw synthesis gas, splitting thisraw synthesis gas into a first and second raw synthesis gas, passing thesecond raw synthesis gas through a pressure swing adsorption (PSA) unit,withdrawing a hydrogen-rich stream and a PSA-off gas stream from the PSAunit;

(e) converting the first raw synthesis gas from step (d) into liquidhydrocarbons through Fischer-Tropsch synthesis;

(f) recycling tail gas from the Fischer-Tropsch synthesis to the feedhydrocarbon stream prior to step (b), to step (b), to step (c1), to step(c2), or combinations thereof.

In one embodiment of the invention in connection with the above or anyof the below embodiments, the feed hydrocarbon stream is subjected to adesulphurization step. In particular, prior to step (b) the feedhydrocarbon stream is subjected to a desulphurization step. Suitably,this desulphurization step is conducted upstream one or morepre-reformers. In the desulphurization step sulphur compounds such asorganic sulphur compounds are removed by conversion with hydrogen tohydrogen sulphide (H₂S) with subsequent absorption on ZnO or othersuitable absorption mass. The desulphurization step enables eliminationof sulphur compounds which are poisonous to catalysts used in downstreamunits, such as pre-reformer catalysts or autothermal reformingcatalysts. The tail gas from the Fischer-Tropsch synthesis which isrecycled to the feed hydrocarbon stream prior to step (b) may thussuitably be recycled to the desulphurization step.

In one embodiment of the invention in connection with any of the aboveor below embodiments, the hydrocarbon-steam stream is pre-reformed inone or more pre-reformers, preferably in one or more adiabaticpre-reformers.

The term “pre-reforming” and “pre-reformer” as used herein shall mean asteam reforming process and steam reformer by which higher hydrocarbonsare converted to a mixture of methane, carbon oxides and hydrogen attemperatures in the range 375-650° C., more specifically 400-600° C.,preferably adiabatically in a fixed bed of catalyst, and its mainpurpose is to remove hydrocarbons higher than methane. Thus, as usedherein “higher hydrocarbons” are hydrocarbons with 2 or more carbonatoms (C₂₊).

As is well known for a person skilled in the art, the term“pre-reforming” shall not be exchanged with steam reforming or otherreforming processes, such as steam methane reforming (SMR), autothermaland/or secondary reforming. Pre-reforming is normally conducted at theabove temperatures (375-650° C.) in a fixed bed of catalyst, and itsmain purpose is to remove hydrocarbons higher than methane, whereassteam methane reforming is a subsequent stage conducted at much highertemperatures (700-1000° C.) and with the main purpose of producing amixture of CO, CO₂ and H₂ (synthesis gas) suitable for downstreamapplications such as Fischer-Tropsch synthesis in large scale plants(e.g. above 5000 BPD).

In another embodiment in connection with any of the above or belowembodiments, step (a) further comprises using Vacuum Pressure SwingAdsorption (VPSA). Accordingly, the oxygen with purity of at least 80%vol. is provided by combining cryogenic ASU and VPSA. By combining theseunits it is possible to reduce the capital costs associated with theprovision of the oxygen. VPSA units are also simpler than ASU units andalso smaller in size.

In yet another embodiment in connection with any of the above or belowembodiments, the hydrogen-rich stream of step (d) is used inhydroprocessing units downstream, such as hydrocracking or hydrotreatingunits in the upgrading section of the Fischer-Tropsch synthesis, or inthe desulphurization step, as described above, or in both. The provisionof the hydrogen-rich stream of step (d) in the desulphurization step,e.g. prior to step (b) enables reducing the size of the desulphurizationsection due to the higher purity of the hydrogen. By way of comparison,a hydrogen stream from H₂-membranes will include CO₂ and H₂O moisturewhich will impact the H₂S absorption and result in undesired H₂Sslippage from the desulphurization step to the pre-reformer(s) and/orautothermal reformer. Contrary to the prior art, particularly if aH₂-membrane were used, the hydrogen removed in the PSA has already anelevated pressure, e.g. 20 atm, for downstream processes such us unitswithin the upgrading section of the Fischer-Trospch section, forinstance hydroprocessing units, more particularly hydrocracking units. Abooster compressor is still needed, but it is much less energy intensiveand thereby much less expensive than compressing from atmosphericpressure. In addition, since the H₂ purity from the PSA is higher than aH₂-membrane, i.e. >99.9% instead of 92-98%, the amount of hydrogenrequired is reduced. This enables additional reduction of highlyexpensive compression requirements and concomitantly better energyefficiency in the process is achieved. For example, hydrogen that comesfrom the PSA is at high pressure, but not the required pressure (i.e. itis 20 bar vs a few bars). A compressor to recycle back to thedesulphurization step upstream may be required as so is a compressor tothe hydrocracking section downstream. Yet, these compressors aresignificantly smaller compared to a situation where a H₂-membrane isutilized.

From the PSA an off-gas is also produced. This off-gas is normally atlow pressure and used as fuel.

In yet another embodiment in connection with any of the above or belowembodiments, the feed hydrocarbon stream is natural gas, associated gas,or combinations thereof. While associated gas is known in particular tocontain a significant amount of higher hydrocarbons (C₂₊), natural gasdespite having a lower amount of higher hydrocarbons, may also benefitfrom having these removed, preferably by use of pre-reforming asmentioned above.

In yet another embodiment in connection with any of the above or belowembodiments, steam is added to the oxygen of step (a). Hence, the oxygenfrom the cryogenic ASU optionally including VPSA, is mixed with steamprior to entering the ATR or CPO. The steam serves to dilute the oxygento the required levels in the ATR thereby providing safety and furtherenabling use of inexpensive construction materials.

It would be appreciated that the oxygen purity obtained from thecryogenic ASU or VPSA may vary. For instance the cryogenic ASU mayprovide oxygen with purity of 90% vol. or more, e.g. 95% vol. or more,while the VPSA may provide oxygen with purity of 80% vol. or more, e.g.90-92% vol. The provision of oxygen with purity of at least 80% vol.,preferably at least 80% vol., more preferably at least 90% vol., mostpreferably at least 95% vol., may result from combining the streams fromboth units.

In a particular embodiment in connection with any of the above or belowembodiments, in step (d) the raw synthesis is dewatered prior to passingto the PSA unit. This enables removal of water which is undesired in theFischer-Tropsch reaction. Suitably, the entire raw synthesis gas isdewatered prior to splitting this raw synthesis gas into a first andsecond raw synthesis gas. Hence, according to this embodiment, the rawsynthesis gas (from the ATR or CPO) is cooled in one or more heatexchangers to recover heat and passed to process condensate separator toremove water (dewatering), prior to splitting this raw synthesis gasinto said first and second raw synthesis gas.

It would be understood by the skilled person that the phase separationis not necessarily 100% and thus some H₂O moisture (i.e. 0-1% vol.) maybe present in the dewatered stream. This depends on the cooling mediaused (air or water) and thereby the separation temperature.

It would also be understood that the term “raw synthesis gas” means theeffluent gas from the ATR or CPO, or the effluent gas from the ATR orCPO which has passed through a water separating step in a processcondensate separator and thus has been dewatered, as described above.

It is also apparent that by the present invention there is no water gasshift stage such as high temperature shift, medium temperature shift,low temperature shift, or combinations thereof, for treating the rawsynthesis gas (from the ATR or CPO) upstream the PSA.

The process is particularly suitable for small GTL plants, i.e. capableof producing 500-5000 BPD of liquid hydrocarbons, suitably 500-3000 BPD.

In a second aspect of the invention there is provided a plant forproducing liquid hydrocarbons from a feed hydrocarbon stream comprisinga reforming section for producing synthesis gas and a Fischer-Trospchsynthesis section for converting the synthesis gas into liquidhydrocarbons, in which said Fischer-Trospch synthesis section includesan upgrading section from which said Fischer-Tropsch tail gas isproduced, wherein the reforming section comprises a cryogenic airseparation unit (ASU) for provision of oxygen with purity of at least80% vol., preferably oxygen with purity of at least 85% vol., morepreferably oxygen with purity of at least 90% vol., most preferablyoxygen with purity of at least 95% vol.; an autothermal reformer (ATR)or catalytic partial oxidation unit (CPO) for producing a raw synthesisgas, means for splitting the raw synthesis gas into a first and secondraw synthesis gas, a pressure swing adsorption (PSA) unit for hydrogenremoval from the second raw synthesis gas, means for passing the firstraw synthesis gas to the Fischer-Tropsch synthesis section, and meansfor recycling the Fischer-Tropsch tail gas to the feed hydrocarbonstream, to the ATR or CPO, or both.

It would be understood by the skilled person that the PSA also producesan off-gas which will be at low pressure and is used as fuel.

In an embodiment in connection with the above or any of the belowembodiments according to the second aspect of the invention, the plantfurther comprises a desulphurization unit for removal of sulphurcompounds from the feed hydrocarbon stream. Suitably thedesulphurization unit comprises a hydrogenation unit which includesmeans for adding hydrogen, and a downstream absorption unit forabsorbing H₂S produced in the hydrogenation unit. The absorption unit issuitably provided as a vessel containing a fixed bed of ZnO or any otherabsorption mass.

The tail gas from the Fischer-Tropsch synthesis which is recycled to thefeed hydrocarbon stream may thus suitably be recycled to thedesulphurization step.

Suitably, the feed hydrocarbon stream is natural gas, associated gas, orcombinations thereof.

In a particular embodiment in connection with any of the above or belowembodiments according to the second aspect of the invention, the plantfurther comprises means for adding steam: to the feed hydrocarbon streamto form a hydrocarbon-steam stream, to the oxygen with purity of atleast 80% vol., or both.

In a particular embodiment in connection with any of the above or belowembodiments according to the second aspect of the invention, the plantfurther comprises one or more pre-reformers, preferably adiabaticpre-reformers, for removal of higher hydrocarbons (C₂₊) from thehydrocarbon-steam stream.

In yet a particular embodiment in connection with any of the above orbelow embodiments according to the second aspect of the invention, theplant further comprises using Vacuum Pressure Swing Adsorption (VPSA)for the provision of the oxygen with purity of at least 80% vol.

As with the first aspect of the invention, it would be appreciated thatthe oxygen purity obtained from the cryogenic ASU or VPSA may vary. Forinstance, the cryogenic ASU may provide oxygen with purity of 90% vol.or more, e.g. 95% vol. or more, while the VPSA may provide oxygen withpurity of 80% vol. or more, e.g. 90-92% vol. The provision of oxygenwith purity of at least 80% vol., preferably at least 85% vol., morepreferably at least 90% vol., most preferably at least 95% vol., mayresult from combining the streams from both units.

In yet another particular embodiment in connection with any of the aboveembodiments according to the second aspect of the invention, the plantfurther comprises means for using the hydrogen removed in the PSA unitin the upgrading section of the Fischer-Tropsch section, or in thedesulphurization unit, or both. The provision of the hydrogen removed inthe PSA unit in the desulphurization unit enables reduction of size insuch unit and thereby reduced capital expenses section due to the higherpurity of the hydrogen, e.g. 99.9% purity. In addition, there is areduction in the compression requirements for the provision of thehydrogen to the desulphurization unit or the upgrading section of theFischer-Tropsch section for instance hydroprocessing units, likehydrotreating units, more particularly hydrocracking units therein.

In yet another embodiment in connection with any of the above or belowembodiments, the plant further comprises upstream said means forsplitting the raw synthesis gas: one or more heat exchangers for coolingthe raw synthesis gas, optionally an air cooler, and a processcondensate separator to remove water from the thus cooled raw synthesisgas. This enables removal of water which is undesired in theFischer-Tropsch reaction.

As with the first aspect of the invention, the term “raw synthesis gas”means the effluent gas from the ATR or CPO, or the effluent gas from theATR or CPO which has passed through a water separating step in theprocess condensate separator and thus has been dewatered, as describedabove.

It is also apparent that there is no water gas shift stage such as hightemperature shift, medium temperature shift, low temperature shift, orcombinations thereof, for treating the raw synthesis gas (from the ATRor CPO) upstream the PSA.

The plant is particularly suitable for small GTL plants, i.e. capable ofproducing 500-5000 BPD of liquid hydrocarbons, suitably 500-3000 BPD.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE shows a schematic representation of a process and plantfor small scale GTL according to the present invention with tail gasrecycle from the Fischer-Tropsch synthesis to autothermal reformerand/or to the desulfurization unit as well as recycle of hydrogen-richstream from the PSA unit to the desulfurization unit.

DETAILED DESCRIPTION OF THE INVENTION

In the FIGURE, a schematic process and plant 1 for producing about 3000BPD of liquid hydrocarbon product is shown. A feed hydrocarbon streamsuch as natural gas 2 is passed through a desulfurization unit 30suitably arranged as a hydrogenator followed by an absorption unit (notshown). To the natural gas 2 or to the desulfurization unit 30, morespecifically to the hydrogenator therein, a hydrogen-rich stream 3 fromthe PSA-unit downstream is added. The desulfurized stream 4 is thenmixed with steam and pre-reformed in one or more pre-reformers (notshown) before entering an autothermal reformer (ATR) 40 under theaddition of oxygen 5. A raw synthesis gas 6 is withdrawn from the ATR,cooled in heat exchangers and air cooler (not shown) before passing to awater removal unit 50 such as a process condensate separator. A largeportion of water is removed from this unit and the raw synthesis gas 6,now dewatered, is split into a first raw synthesis gas 7 whichrepresents the major portion and second raw synthesis gas 8 whichrepresents the minor portion of the raw synthesis gas 6. The first rawsynthesis gas 7 is then converted to liquid hydrocarbon product byFischer-Tropsch synthesis which includes an upgrading section (notshown) from which tail gas 9 is recycled to the hydrocarbon feed 2 (notshown), to the desulfurization unit as shown here, to the pre-reformers(not shown), or to the autothermal reformer 40, e.g. by adding tail gas9 to the hydrocarbon stream entering the autothermal reformer. Thesecond raw synthesis gas 8 is passed through a Pressure Swing Adsorption(PSA) unit 60 out of which a PSA off-gas 10 is produced and used as fuelas well as a hydrogen-rich stream 11 which can be diverted ashydrogen-product stream 12 due to its high purity, e.g. 99.9% hydrogen.A hydrogen recycle stream 3 is used in the hydrogenator ofdesulfurization unit 30 and suitably also in downstream hydrocrackingunits of the upgrading section of the Fischer-Tropsch synthesis (notshown).

The invention claimed is:
 1. A process for the conversion of a feedhydrocarbon stream into liquid hydrocarbons comprising: (a) producingoxygen with a purity of at least 80% vol. in at least a vacuum pressureswing adsorption (VPSA) unit; (b) mixing steam to the feed hydrocarbonstream to form a hydrocarbon-steam stream; (c1) adding the oxygen ofstep (a) and the hydrocarbon-steam stream of step (b) to an autothermalreformer (ATR), or catalytic partial oxidation (CPO) unit, or (c2)combining the oxygen of step (a) with the resulting stream of step (b)and then adding to an autothermal reformer (ATR), catalytic partialoxidation (CPO) unit; (d) withdrawing from the ATR or CPO a rawsynthesis gas which is first dewatered and then split into a first andsecond raw synthesis gas stream; (e) removing hydrogen from the secondraw synthesis gas stream without any water-gas-shift pre-treatment, andwithdrawing a hydrogen-rich stream and an off gas stream; (f) convertingthe first raw synthesis gas stream from step (d) into liquidhydrocarbons through Fischer-Tropsch synthesis and a tail-gas; (g)recycling tail gas from the Fischer-Tropsch synthesis to the feedhydrocarbon stream prior to step (b), to step (b), to step (c1), to step(c2), or combinations thereof.
 2. The process according to claim 1,wherein, prior to step (b), the feed hydrocarbon stream is subjected toa desulphurization step.
 3. The process according to claim 1, whereinthe hydrocarbon-steam stream is pre-reformed in one or morepre-reformers.
 4. The process according to claim 1, wherein saidproducing oxygen with a purity of at least 80% vol. includes cryogenicseparation unit.
 5. The process according to claim 1, wherein the secondraw synthesis gas is fed to a PSA unit to produce said hydrogen-richstream.
 6. The process according to claim 1, wherein the feedhydrocarbon stream is natural gas, associated gas, or combinationsthereof.
 7. The process according to claim 1, wherein steam is added tothe oxygen of step (a).
 8. A process for the conversion of a feedhydrocarbon stream into liquid hydrocarbons comprising: (a) producingoxygen with a purity of at least 80% vol. in at least a vacuum pressureswing adsorption (VPSA) unit; (b) mixing steam to the feed hydrocarbonstream to form a hydrocarbon-steam stream; (c1) adding the oxygen ofstep (a) and the hydrocarbon-steam stream of step (b) to an autothermalreformer (ATR), or catalytic partial oxidation (CPO) unit, or (c2)combining the oxygen of step (a) with the resulting stream of step (b)and then adding to an autothermal reformer (ATR), catalytic partialoxidation (CPO) unit; (d) withdrawing from the ATR or CPO a rawsynthesis gas which is first dewatered and then split into a first rawsynthesis gas stream, and a second raw synthesis gas stream; (e)removing hydrogen from the second raw synthesis gas stream without anywater-gas-shift pre-treatment, and withdrawing a hydrogen-rich streamand an off gas stream; (f) converting the first raw synthesis gas streamfrom step (d) into liquid hydrocarbons through Fischer-Tropsch synthesisand a tail-gas; (g) recycling tail gas from the Fischer-Tropschsynthesis to the feed hydrocarbon stream prior to step (b), to step (b),to step (c1), to step (c2), or combinations thereof.